1. Field of the Invention
The present invention relates to a zoom lens. More specifically, the present invention relates to a zoom lens useful as a photographic optical system for an optical apparatus, such as a digital still camera, a video camera, a TV camera, or a monitoring camera.
2. Description of the Related Art
In recent years, the functions of an optical apparatus (image pickup apparatus (camera)) that uses a solid-state image sensor, such as a video camera or a digital still camera, have increased substantially. At the same time, market forces demand that newly designed image pickup apparatuses be small and highly efficient. To that end, an optical system used in an optical apparatus like a camera is designed with a small-size zoom lens having a wide angle of view (photographic angle of view), a high aperture ratio, and a high optical performance.
A camera of this type includes various optical members, such as a low-pass filter and a color correction filter, which are disposed between a rearmost lens and an image sensor. Accordingly, it is desirable that a zoom lens used as an optical system of the camera described above has a relatively long back focal length. In addition, for a color camera that uses an image sensor for capturing a color image, it is desirable that the optical system of the camera has a high telecentricity on the image side in order to prevent shading of color.
As a zoom lens (optical system) whose entire size is small and which has a long back focus and a high telecentricity on the image side, a negative lead type zoom lens is conventionally used. In the negative lead type zoom lens, a lens unit having a negative refractive power is located at a position closest to the object side.
U.S. Pat. No. 6,646,815 (Patent '815) discusses an example of a negative lead type zoom lens. Patent '815 discloses a three-unit zoom lens including, in order from the object side to the image side, a first lens unit having a negative refractive power, a second lens unit having a positive refractive power, and a third lens unit having a positive refractive power, and having a high telecentricity on the image side. Similarly, U.S. Pat. No. 7,474,472 (Patent '472) discusses a three-unit zoom lens of this zooming type having a high telecentricity on the image side and a high aperture ratio.
In a negative lead type zoom lens including three or more lens units, in order to achieve a wide angle of view and a high zoom ratio while reducing the size of the entire optical system, it is useful to increase the refractive power of each lens unit. However, if the refractive power of each lens unit is merely increased to achieve a wide angle of view and a high zoom ratio, the amount of aberrations occurring during zooming may greatly vary. Accordingly, in this case, it may become difficult to achieve a high optical performance for the entire zoom range.
Furthermore, if the aperture ratio is increased at the wide-angle end in this case, it becomes difficult to correct various aberrations including spherical aberration and coma in particular. In order to effectively correct various aberrations in this case, it is necessary to increase the number of lenses included in each lens unit. However, if the number of lenses is merely increased, the lens total length may increase. Accordingly, in this case, it becomes difficult to reduce the total size of the optical system.
In a negative lead type zoom lens including three or more lens units, in order to achieve a wide angle of view, a high zoom ratio, and a wide aperture ratio while reducing the size of the entire optical system, it is significant to satisfy at least one of the following conditions. Firstly, it is necessary to appropriately set the refractive power of the first lens unit in relation to the refractive power of the entire optical system. If the refractive power of the first lens unit is not appropriately set in relation to the refractive power of the entire optical system, the effective diameter of the front lens becomes very large. In this case, the size of the entire optical system cannot be easily reduced.
Secondly, it is necessary to appropriately set the arrangement of the refractive power of each lens included in each lens unit located around the aperture stop. If the arrangement of the refractive power of each lens included in each lens unit located around the aperture stop is not appropriately set, it becomes difficult to effectively correct spherical aberration and coma at the wide-angle end.
Thirdly, it is necessary to appropriately set a condition for moving a second lens unit and a third lens unit during zooming. If a condition for moving a second lens unit and a third lens unit during zooming is not appropriately set, it becomes difficult to effectively correct spherical aberration and coma at the wide-angle end.